This invention relates to a power source for a nuclear fusion reactor and, more particularly, to a power source for a coil provided in a nuclear fusion reactor such as a reversed field pinch (RFP) apparatus.
Heretofore, as a prior-art power source of this type is known a constitution shown in FIG. 1. The power source will be described as an example with respect to a power source used for a nuclear fusion reactor such as a reversed field pinch (RFP) apparatus.
In FIG. 1, reference numeral 1 designates a power source, numeral 2 designates a capacitor Cr, numeral 3 designates a capacitor Cf, numerals 4 and 5 designate diodes D.sub.1 and D.sub.2, numeral 6 designates a resistor R, numeral 7 designates an ignitron Ig for forming a main circuit switch, and numeral 8 designates a load coil L.
In such a prior-art power source, the capacitors Cr 2 and Cf 3 are now charged by a charger (not shown) to predetermined voltages, respectively. For example, the former is charged to 20 kV, and the latter is charged to 3.3 kV. When a trigger signal is applied from a firing circuit (not shown) to the ignitron Ig 7, the ignitron Ig 7 is fired. Thus, a current I from the capacitor Cr 2 starts flowing to the load coil L 8. FIG. 2 shows a diagram of the relationship between the time and the load current illustrating the state that the current I alters with the time. This operation will be continuously explained with reference to FIG. 2. When the current I flows in some degree, the voltage of the capacitor Cr 2 decreases. When the voltage of the capacitor Cr 2 becomes equal to the voltage of the capacitor Cf 3, the current from the capacitor Cf 3 is applied through a diode D.sub.1 4, and flowed to the load coil L 8. This state is shown by a time point t.sub.r in FIG. 2. The current I then continues increasing and arrives at the peak value I.sub.0 at a time point t.sub.0. After the time point t.sub.0, the currents from the capacitors Cr 2 and Cf 3 are eliminated, but a clover circuit is formed by the resistor 6, the diodes D.sub.2 and D.sub.1, the ignitron Ig 7 and the load coil L 8, and the current I flowed to the load coil L 8 continues flowing while gradually decreasing. The resistor R 6 is one type of a current limiting resistor inserted to restrict the allowable capacity of the ignitron Ig 7. Thus, in the prior-art power source in FIG. 1, the current change as shown by a solid line in FIG. 2 occurs.
In FIG. 2, the load current I in the time zone of t=0 to t.sub.r is that from the capacitor Cr 2, and the load current I in the time zone of t=t.sub.r to t.sub.0 is that from both the capacitors Cr 2 and Cf 3. The current flowed after the time point t.sub.0 is the current of the clover circuit, which flows through the clover circuit formed as described above.
The ignitron used in the prior-art power source of this type has a small current capacity an as high as approx. 100 coulomb per one even in the largest due to the restriction in the structure. Thus, the value of the resistor R of the clover circuit is set largely so as to rapidly decrease the current of the clover circuit for the load coil. However, since the early reduction in the current of the load coil shortens the time for closing a plasma generated in the nuclear fusion reactor, it is not preferable. Then, the resistor of the clover circuit is reduced so as to delay the reduction in the current of the clover circuit, and it is considered to connect a number of ignitrons in parallel so as to increase the current capacity of the ignitron. For example, when the ignitron is adopted as a power source for a RFP apparatus, approx. 10 ignitrons are connected in parallel. However, in such a case, a problem arises that gate circuits and DC power sources for the ignitrons are necessarily increased that much, thereby causing the cost to increase and the profile to be increased in size. Further, it is also considered to utilize SCRs as a main circuit switch, but since the voltage of the capacitor in the circuit is high, another problem arises that it is necessary to use several parallel connections of SCRs connected in series, thereby causing the cost to increase.